Pacemaker rate control is conventionally derived from control signals obtained from a plurality of measuring elements such as cardiac catheters, special breathing sensors, body temperature sensors, etc. Functional parameters used for the control of the pacing rate are dependent upon a patient's physical condition and dynamically changing exercise parameters. It is desirable therefore to have the pacing rate controlled by information derived from a plurality of physiological parameters of the patient.
Some available publications describe pacing rate control of a pacemaker by measured signals based on the detection of one physiological functional parameter to provide pacing rate control dependent upon pulmonary activity. Thus, in U.S. Pat. No. 4,567,892, G. Plicchi, et. al., Feb. 4, 1986, the respiratory rate is determined from an implanted secondary electrode by an impedance measurement. In U.S. Pat. No. 4,697,591, A. Lekholm, et al., Oct. 6, 1987, the respiratory rate is determined from impedance across the chest cavity by using the can and heart implant electrodes. In U.S. Pat. No.4,596,251, G. Plicchi, et al. , June 24, 1986, the respiratory minute volume is measured by impedance changes from at least one electrode located in the chest cavity. Other related respiratory rate controls are effected in U.S. Pat. Nos. 3,593,718, J. L. Krasner et al., July 20, 1971; 4,721,110, M. S. Lampadius, Jan. 26, 1988 and 4,702,253, T. A. Nappholz et al., Oct. 27, 1987. In U.S. Pat. No. 4,576.183 G. Plicchi, et al., Mar. 18, 1986 subcutaneous electrodes in a patient's chest are used to measure impedance for control by a respiratory parameter.
Recently there have also been proposals to control the pacing rate of a cardiac pacemaker from two or more physiological functional parameters. In German Pat. P 36 31 155C, published Mar. 24, 1988, pacing rate is controlled for stable long-term control from the temperature of the venous blood within the heart and from an activity sensor for short-term exercise related activity. The temperature signals can be modulated by the activity signals for an optimal adaptation of the pacing rate to the particular exercise of the patient. Different sensors may be used to check the two functional parameters. The pacemaker control is based on the finding that essentially only parameters such as the blood temperature and activity should be used as absolute values for determining a relationship between these parameters and the pacing rate, whereas other physiological functional parameters are merely relative parameters, which at least impede stable long-term control of the pacemaker. U.S. Pat. No. 4,722,342, D. Amundson, Feb. 2, 1988provides a plurality of different body activity sensors to derive variable pacer controls for body activity. Respiratory control of a pacemaker pulse rate with a respiratory signal derived from analyzing the stimulation pulse reaction on the already implanted pacemaker electrode is set forth in U.S. Pat. No. 4,694,830 issued to A. Lekholm Sept. 22, 1987.
The first generation of rate responsive cardiac pacemakers used only one parameter to control the rate of pacing. In U.S. Pat. No. 4,527,568 of A. Rickards, July 9, 1985, the change of QT interval with exercise is proposed for rate control. With that parameter it is not possible to establish an absolute relation between QT interval and heart rate. Therefore, a rate responsive pacemaker can use this parameter for rate control only if relative changes of the parameter are applied to a self-established relative baseline value. More advanced concepts of rate control using the QT interval and adjusting the slope automatically on the measured QT interval are disclosed in Vitatron Medical: Clinical Evaluation Report Model 919, Aug. 1988, Velp. Even though this represents an advanced concept, it suffers from considerable drawbacks. Since the QT interval as the control parameter depends on paced heart beats, correct adjustments are not achievable in those patients that feature intrinsic heart beats. This makes possible the automatic gain setting only in those patients that depend on continuously paced heart beats. As an in built 24 hour clock is additionally needed to define night time (assumed to be the resting time) for these measurements, difficulties arise with those patients that are active at night or that travel with differences in local time.
As far as the relativity of an parameter is concerned, the same holds true for several other parameters that have been proposed for control as in U.S. Pat. Nos. 4,535,774, W. H. Olson, Aug. 20, 1985; 4,674,518, R. W. Salo, June 23, 1987; and 4,566,456, G. Koning, et al., Jan. 28, 1986. Both techinical limitations and underlying human physiology prevents satisfactory operation of such systems. Use of blood temperature for control of rate responsive cardiac pacing is proposed in U.S. Pat. Nos. 4,436,092, Mar. 13, 1984; 4,543,954, Oct. 1, 1985; and 4,719,920, Jan. 19, 1988. The time delays imposed by temperature changes make real time response to sensed physiological parameters in a patient more difficult.
The prior art in general has assumed that an absolute relation between a physiological parameter and the pacing rate should fit different metabolic conditions of the patient. However the prior art does not custom fit or tailor the pacing rate with a plurality of sensed physiological parameters to offset individual patient response to variable conditions such as exercise, nor adjust the pacemaker rate response to exercise in accordance with the different basic conditions of the various individual patients.
Furthermore such prior pacemaker controls are generally deficient in their controls of pacing in response to a patient's condition because of measurement errors attributed to interference between pacing pulses and electric sensor pulses, or to errors induced by improper ventilation signals derived from a plurality of sensor electrodes and sensor electrodes located where false signals are given from body motion, coughing or the like.
None of the prior art therefore could reliably provide dynamic adjustments of the pacing rate to fit the characteristic unique needs of an individual patient, particularly since there is no uniformity of response of patients to different work loads, for example.
Therefore, it is an object of this invention to provide more reliable determination of a patient's individual physiological parameters and basic condition to use them for automatically adjusting the pacing pulses to an individually tailored optimum rate for both rest and exercise.
A further object of this invention is to provide control of pacing rates tailored to respond to individual exercise and respiratory characteristics of a patient.